A common requirement for an advanced electronic circuit and particularly for circuits manufactured as integrated circuits (“ICs”) in semiconductor processes is the use of varactors. Varactors or “variable reactors” provide a voltage controlled capacitor element that has a variable capacitance based on the voltage expressed at the terminals and a control voltage. Metal oxide semiconductor or MOS varactors may have a control voltage applied to a gate terminal that provides a control on the capacitance obtained for a particular voltage on the remaining terminals of the device.
Because a varactor is based on a reverse biased P-N junction, the terminals are typically biased such that no current flows across the junction. A circuit element structure where no current flows between the terminals provides, in essence, a capacitor. However, by varying the bias on the third terminal (the “gate” for a MOS varactor), the device may form a depletion or even an accumulation region under the gate, changing the current flow through the device. The effective capacitance obtained is thus variable, and, voltage dependent. This makes the varactor very useful as a voltage controlled capacitor. This circuit element is particularly useful in oscillators, RF circuits, mixed signal circuits and the like.
The capacitance obtained at a given control voltage for a varactor is dependent on physical quantities including the gate oxide thickness (“Tox”) and the doping of, for example, the doped well the varactor is formed in. MOS varactors may be an N+/n well type, a P+/p-well type, for example. The well doping concentration and Tox are both physical factors that may be determined by using the measured varactor capacitance observed. These characteristics make varactors very useful as process control monitors (“PCM”s) in semiconductor fabrication. At a wafer acceptance test (“WAT”) stage, measurements of a varactor formed as a test structure or PCM on the wafer can provide quality information about the Tox and the well doping characteristics of the wafer. Bad lots can quickly be identified and other wafers can be “binned” as better, or less better, lots based on the results of the WAT.
Further, because varactors offer a tunable capacitance, they are often used as circuit elements in radio frequency (“RF”) and mixed signal circuit devices such as voltage controlled oscillators (“VCOs”), pulse controlled modulators (“PCMs”), delay lines, and the like. Of additional importance is the CV curve performance at various frequencies; for example, RFs are particularly important for semiconductor devices that might be used for forming circuits for cellphones or other wireless or radio components.
A continuing need thus exists for a MOS type varactor that is compatible with advanced semiconductor processes without the need for additional process steps, a varactor that is scalable across semiconductor process technology nodes, and which provides consistent performance compatible with modeling and circuit simulation across baseband, RF, MS and other frequencies, with an extended tuning ratio and which is useful as a PCM without the need for manual calibration steps after manufacture.
The drawings, schematics and diagrams are illustrative and not intended to be limiting, but are examples of embodiments of the invention, are simplified for explanatory purposes, and are not drawn to scale.